1. Field of the Invention
The present invention relates to a car battery system with battery state detection circuits connected to battery blocks having a plurality of battery cells connected in a stacked fashion.
2. Description of the Related Art
A car battery system has many battery cells stacked together and connected in series to increase the output voltage. In this type of battery system, battery cell degradation is prevented by controlling battery charging and discharging while detecting the state of each battery cell. Each battery cell connected in series is charged and discharged by the same current. Remaining battery capacity is computed by integrating battery charging and discharging currents, and charging and discharging is controlled to keep the remaining capacity within a set range. Remaining capacity is computed by adding integrated charging current values and subtracting integrated discharging current values. In practice, actual remaining capacity differences develop over time for battery cells charged and discharged by the same currents. This is because differences in battery cell temperature and electrical characteristics cause variation in the actual charging and discharging of each battery cell. If differences in actual remaining capacity develop, a battery cell with low remaining capacity can easily be over-discharged while a battery cell with high remaining capacity can easily be over-charged, and this can be the cause of battery cell degradation. This is because a battery cell can be significantly degraded by over-charging or over-discharging. Since a car battery system is provided with many battery cells, manufacturing cost is extremely high and extending system life-time is of utmost importance.
Battery cell degradation can be prevented by detecting the voltage of each battery cell and controlling actual remaining battery capacity to keep it in a set range. Therefore, in a battery system having battery blocks with many series-connected battery cells, battery state detection circuits are provided to detect the voltage of each battery cell. These battery state detection circuits are disposed next to the battery blocks and are connected to the positive and negative electrode terminals of each battery cell via a wire-harness (refer to Japanese Patent Application Disclosure 2008-140631).
As shown in the abbreviated view of FIG. 1 for example, a car battery system with many battery cells stacked together has a wire-harness 94 connected to the electrode terminals 93 of each battery cell 91, and this wire-harness 94 is connected to a battery state detection circuit 90 disposed outside the battery block 92. The wire-harness 94 has many long wire-leads 95 bundled together, and each wire-lead 95 is connected to the electrode terminals 93 of each battery cell 91 and to the battery state detection circuit 90. Not only is the wire-harness 94 long, but the lengths of the wire-leads 95 connecting different battery cells 91 are different. For example, wire-leads 95 connecting battery cells 91 furthest from the from the battery state detection circuit 90 are extremely long. In a long wire-harness with different length wire-leads, impedances of the wire-leads are high and there can be significant impedance differences for different wire-leads. Differences in wire-lead impedances can be the cause of battery cell voltage detection error in the battery state detection circuit. In particular, it is necessary for the battery state detection circuit to detect voltage differences between battery cells with extremely high precision. For example, a battery system with lithium-ion battery cells demands a very high individual battery cell voltage detection accuracy of 0.05 V or better, and preferably 0.02 V or better. Since prior art battery systems employ wire-harnesses with very long wire-leads bundled together, the large impedance of the wire-harness has become a cause of reduced individual battery cell measurement accuracy.
Further, since the wire-harness of a prior art battery system bundles together wire-leads connected to each battery cell, the battery system has the drawbacks that an open circuit in the wire-harness can cause functional failure and a short circuit between wires in the wire-harness can cause fire or smoke.